HIGH-STRENGTH HOT-ROLLED STEEL PLATE WITH GOOD DUCTILITY, STRETCH FLANGEABILITY AND MATERIAL QUALITY UNIFORMITY, AND PROCESS FOR MANUFACTURING SAME

Abstract

 The present invention provides a hot-rolled steel sheet having high strength and excellent ductility and stretch flangeability, and good uniformity of material in which the variation in strength in a coil is small, and a method for manufacturing the same. A slab having a steel composition including 0.020% to 0.065% of C, 0.1% or less of Si, 0.40% to less than 0.80% of Mn, 0.030% or less of P, 0.005% or less of S, 0.08% to 0.16% of Ti, 0.005% to 0.1% of Al, 0.005% or less of N, and the balance being Fe and incidental impurities, in which Ti* (= Ti - (48/14) x N) satisfies [TiC* ≥ 0.08] and [0.300 ≤ C/Ti* ≤ 0.375], is subjected to hot rolling to obtain a hot-rolled steel sheet in which the steel microstructure includes, in terms of area fraction, 95% or more of a ferrite phase; the average ferrite grain size is 10 µm or less; the average grain size of Ti carbides precipitated in steel is 10 nm or less; and Ti in the amount of 80% or more of Ti* is precipitated as Ti carbides.

Description

[Technical Field]

[0001] The present invention relates to a high strength hot-rolled steel sheet useful for frame components for large-sized vehicles and automobiles, such as frames for trucks.

[Background Art]

[0002] In recent years, from the viewpoint of global environmental protection, in order to control CO₂ emissions, there has been an urgent need to improve the fuel efficiency of automobiles and there has been a demand for weight reduction by reducing the thickness of materials to be used. However, such a reduction in thickness degrades crashworthiness. Since there has also been a requirement to improve safety in order to ensure the safety of occupants at the time of a vehicular collision, it is essential to increase the strength of materials to be used in order to achieve reduction in thickness.

[0003] Many of automobile components for which a steel sheet is used as a material are manufactured by press forming. In general, by increasing the strength of a steel sheet, ductility, stretch flangeability, and the like are degraded and springback is increased. Therefore, formability and shape stability remain problems to be solved. In recent years, it has become possible to predict the amount of springback with high accuracy by CAE (Computer Assisted Engineering). When there is a large variation in material quality, the accuracy of prediction by CAE is deteriorated. Therefore, there has been a demand for a high strength steel sheet having, in addition to formability, excellent uniformity of material in which the variation in strength is small.

[0004] Currently, development has been actively conducted to achieve both high strength and good formability. For example, Patent Literature 1 discloses a high strength hot-rolled steel sheet having excellent ductility, stretch flangeability, and tensile fatigue with a TS of 780 MPa or more, which has a chemical composition including, in percent by mass, 0.06% to 0.15% of C, 1.2% or less of Si, 0.5% to 1.6% of Mn, 0.04% or less of P, 0.005% or less of S, 0.05% or less of Al, 0.03% to 0.20% of Ti, and the balance being Fe and incidental impurities, which has a microstructure including 50% to 90% of a ferrite phase, in terms of volume fraction, and the balance being substantially a bainite phase, the total volume fraction of the ferrite phase and the bainite phase being 95% or more, in which precipitates containing Ti are precipitated in the ferrite phase, and the average diameter of the precipitates is 20 nm or less, and in which 80% or more of the Ti content in the steel is precipitated.

[0005] Furthermore, Patent Literature 2 discloses a high strength hot-rolled steel sheet having excellent stretch flange formability with a TS of 690 to 850 MPa and a λ of 40% or more, which has a chemical composition including, in percent by mass, 0.015% to 0.06% of C, less than 0.5% of Si, 0.1% to 2.5% of Mn, 0.10% or less of P, 0.01% or less of S, 0.005% to 0.3% of Al, 0.01% or less of N, 0.01% to 0.30% of Ti, 2 to 50 ppm of B, and the balance being Fe and incidental impurities, in which the relationships 0.75 < (C%/12)/(Ti%/48)-N%/14-S%/32) < 1.25 and 1.0 < (Mn%+Bppm/10-Si%) are satisfied, the total area fraction of ferrite and bainitic ferrite phases is 90% or more, and the area fraction of cementite is 5% or less.

[0006] Patent Literature 3 discloses a high tensile strength hot-rolled steel sheet having high formability and excellent uniformity of material with a TS of 610 to 830 MPa, which contains, in percent by mass, 0.1% or less of C, 0.05% to 0.6% of Mo, and 0.02% to 0.10% of Ti, in which carbides containing Ti and Mo in the range satisfying the atomic ratio Ti/Mo ≥ 0.1 are dispersed and precipitated in the microstructure including a ferrite structure.

[0007]  Furthermore, Patent Literature 4 discloses a high strength hot-rolled steel sheet having excellent uniformity of strength with a small variation in strength with a TS of 540 to 780 MPa, which has a chemical composition including, in percent by mass, 0.05% to 0.12% of C, 0.5% or less of Si, 0.8% to 1.8% of Mn, 0.030% or less of P, 0.01% or less of S, 0.005% to 0.1% of Al, 0.01% or less of N, 0.030% to 0.080% of Ti, and the balance being Fe and incidental impurities,
in which the area fraction of a polygonal ferrite phase is 70% or more, and the amount of Ti present in precipitates with a size of less than 20 nm is 50% or more of the value of Ti* calculated by the expression [Ti* = [Ti]-(48/14)x[N]].

[0008] However, in the high strength hot-rolled steel sheet described in Patent Literature 1, hard ferrite and bainite phases are required to be produced at specified volume fractions. Since the transformation behavior is not constant with respect to the chemical composition of steel, there is a problem that controlling is difficult during air-cooling for promoting the ferrite transformation. In the high strength hot-rolled steel sheet described in each of Patent Literatures 2 and 3, elongation El is low, and it is not necessarily possible to obtain a steel sheet having good stretch flangeability and material stability, which is a problem. In the high strength hot-rolled steel sheet described in Patent Literature 4, a TS of 590 MPa or more is obtained by solid-solution strengthening using Mn. However, in solid-solution strengthening, the strengthening ratio relative to the amount of the element added is smaller than that in precipitation strengthening using Ti, and thus cost performance is poor. Furthermore, since the amount of C added is large relative to Ti, formation of hard cementite is unavoidable. Therefore, stretch flangeability is poor, which is also a problem.

[Citation List]

[Patent Literature]

[0009] 

[PTL 1] Japanese Unexamined Patent Application Publication No. 2007-9322

[PTL 2] Japanese Unexamined Patent Application Publication No. 2007-302992

[PTL 3] Japanese Unexamined Patent Application Publication No. 2002-322541

[PTL 4] Japanese Unexamined Patent Application Publication No. 2009-185361

[Summary of Invention]

[Technical Problem]

[0010] In order to solve the problems described above, it is an object of the present invention to provide a hot-rolled steel sheet having high strength and excellent ductility and stretch flangeability, and having good uniformity of material in which the variation in strength in a coil is small, and a method for manufacturing the same.

[Solution to Problem]

[0011] The present inventors have performed thorough studies on the intended high strength hot-rolled steel sheet, and as a result, have obtained the following findings.

1) Upon optimization of a chemical composition for the purpose of controlling the precipitation efficiency of TiC and the cementite formation amount, by forming a steel microstructure in which the area fraction of the ferrite phase is 95% and the ferrite grain size is 10 µm or less, it is possible to obtain a hot-rolled steel sheet having a tensile strength (TS) of 590 to 780 MPa, a total elongation (El) of 28% or more, and a hole expanding ratio (λ) of 100% or more.

2) In order to improve uniformity of material, it is important to set the ferrite fraction at a certain level in the steel sheet and to suppress coarsening of TiC. Therefore, by setting the content of Mn, which is an austenite former, to 0.4% to 0.8%, which is in restrained condition, it becomes possible to complete the ferrite transformation in a short period of time, and the manufacturing cost can be reduced. In order to achieve a TS of 590 MPa or more, it is necessary to set the content of Ti to be 0.08% to 0.16%. However, when the content of Ti, which is a precipitate-forming element, is high, precipitates are likely to be coarsened, which is a problem. In order to solve this problem, it is important after obtaining precipitates during ferrite transformation to perform coiling at a low temperature. Specifically, the coiling temperature needs to be 560°C or lower.

[0012] The present invention is based on such findings, and in order to solve the problems described above, the following means are employed:

[1] A high strength hot-rolled steel sheet having excellent ductility, stretch flangeability, and uniformity of material, characterized in that the steel sheet has a steel composition including, in percent by mass, 0.020% to 0.065% of C, 0.1% or less of Si, 0.40% to less than 0.80% of Mn, 0.030% or less of P, 0.005% or less of S, 0.08% to 0.20% of Ti, 0.005% to 0.1% of Al, 0.005% or less of N, and the balance being Fe and incidental impurities, in which Ti* specified by the expression (1) below satisfies the expressions (2) and (3) below, and the steel sheet has a steel microstructure including, in terms of area fraction, 95% or more of a ferrite phase and the balance being at least one of a pearlite phase, a bainite phase, and a martensite phase; the average ferrite grain size is 10 µm or less; the average grain size of Ti carbides precipitated in steel is 10 nm or less; and Ti in the amount of 80% or more of Ti* is precipitated as Ti carbides:






where Ti, N, and C represent contents of corresponding elements (percent by mass).

[2] A method for manufacturing a high strength hot-rolled steel sheet characterized by including heating a steel slab having the steel composition described in [1] at a temperature in a range of 1,200°C to 1,300°C, then performing hot rolling at a finishing temperature of 900°C or higher, starting cooling within 2 seconds after the hot rolling at a cooling rate of 30 °C/s or more, stopping cooling at a temperature of 650°C to 750°C, subsequently, after undergoing a natural cooling step for 5 to 20 seconds, performing cooling at a cooling rate of 30°C/s or more, and performing coiling in a coil shape at 560°C or lower.

[Advantageous Effects of Invention]

[0013] According to the present invention, it becomes possible to manufacture a high strength hot-rolled steel sheet having high strength, excellent ductility and stretch flangeability, and good uniformity of material with a small variation in strength in the steel sheet, in which the tensile strength (TS) is 590 to 780 MPa or more, the total elongation (El) is 28% or more, the hole expanding ratio (λ) is 100% or more, and the variation in TS (ΔTS) is 15 MPa or less. The high strength hot-rolled steel sheet of the present invention is suitable for use in structural members, such as pillars and members of automobiles, and frames of trucks.

[Description of Embodiments]

[0014] The present invention will be described in detail below. Note that the unit expressing the content of each element is "percent by mass", and hereinafter, is simply described as "%".

1) Steel composition

[0015] Reasons for limiting the steel composition (chemical composition) in the present invention will be described.

C: 0.020% to 0.065%

[0016] C is an element that forms fine Ti carbides in the ferrite phase, thus contributing to an increase in strength. In order to obtain a hot-rolled steel sheet with a TS of 590 MPa or more, it is necessary to set the C content at 0.020% or more. On the other hand, when the C content exceeds 0.065%, El and λ are degraded, and also the ferrite transformation speed becomes slow, resulting in degradation in uniformity of material. Therefore, the C content is set at 0.020% to 0.065%, preferably 0.020% to 0.055%, and more preferably 0.050% or less.

Si: 0.1% or less

[0017] When the Si content exceeds 0.1%, the Ar₃ point rises excessively, and thus it becomes difficult to obtain a fine and granular microstructure of the ferrite phase. Furthermore, the increase in the Si content leads to degradation in toughness and fatigue properties. Therefore, the Si content is set at 0.1% or less, and preferably 0.05% or less.

Mn: 0.40% to less than 0.80%

[0018] Mn is effective in increasing strength and refining ferrite grains. In order to obtain a hot-rolled steel sheet having a TS of 590 MPa or more and a ferrite grain size of 10 µm or less, it is necessary to set the Mn content at 0.40% or more. On the other hand, when the Mn content is 0.80% or more, the ferrite transformation speed becomes slow, resulting in degradation in uniformity of material. Therefore, the Mn content is set to be 0.40% to less than 0.80%.

P: 0.030% or less

[0019] When the P content exceeds 0.03%, segregation in the grain boundaries becomes marked, resulting in degradation in toughness and weldability. Therefore, the P content is set at 0.03% or less. Desirably, the P content is decreased as much as possible.

S: 0.005% or less

[0020] S forms sulfides with Mn and Ti to degrade stretch flangeability. Therefore, the S content is set at 0.005% or less. Desirably, the S content is decreased as much as possible.

Al: 0.005% to 0.1%

[0021] Al is utilized as a deoxidizing element and is an element effective for improving the steel cleanliness. In order to obtain such an effect, it is necessary to set the Al content at 0.005% or more. On the other hand, an Al content of more than 0.1% is likely to cause surface defects and results in a rise in costs. Therefore, the Al content is set at 0.005% to 0.1%.

N: 0.005% or less

[0022] N is an element that has a strong affinity for Ti, and forms Ti nitrides which do not contribute to strengthening. Consequently, when the N content exceeds 0.005%, a large amount of Ti is required in order to secure the amount of Ti carbides which contribute to strengthening, which results in a rise in costs. Therefore, the N content is set at 0.005% or less. Desirably, the N content is decreased as much as possible.

Ti: 0.08% to 0.20%

[0023] Ti is an important element in the present invention, and precipitates as fine carbides, TiC and Ti₄C₂S₂, with a grain size of less than 10 nm in the ferrite phase during natural cooling (air cooling) subsequent to primary cooling after hot rolling, thus contributing to an increase in strength. In order to achieve a TS of 590 MPa or more, the Ti content needs to be at least 0.08% or more. On the other hand, when the Ti content exceeds 0.20%, it is difficult to dissolve coarse Ti carbides during heating of the slab prior to hot rolling, and it is not possible to obtain fine Ti carbides which contribute to strengthening after hot rolling. Furthermore, during heating of the slab, non-uniform dissolution of Ti carbides is caused, which impairs uniformity of TS in the steel sheet. Therefore, the Ti content is set at 0.08% to 0.20%, preferably 0.08% to 0.16%, and more preferably 0.08% to 0.13%.

[0024] The balance is Fe and incidental impurities.

Expressions (1) to (3)

[0025] As will be described later, in order to obtain a hot-rolled steel sheet having a λ of 100% or more, it is necessary to control the amount of cementite precipitated. Therefore, the present invention utilizes the phenomenon that Ti binds to C to form Ti carbides, such as TiC and Ti₄C₂S₂.

[0026] Consequently, it is necessary to secure the amount of Ti that can form Ti carbides, and Ti* defined by the expression (1) below needs to satisfy the expression (2) below.




Ti* represents the amount of Ti that can form Ti carbides.

[0027] In order to obtain good stretch flangeability, it is necessary to control the amount of cementite. In the steel of the present invention, the amount of excess C that does not form Ti carbides corresponds to the amount of cementite formed. When the amount of cementite formed increases, stretch flangeability tends to degrade. In order to obtain a λ of 100% or more, the (C/Ti*) value needs to be set at 0.375 or less. Furthermore, when this value is less than 0.300, the amount of fine Ti carbides formed is insufficient, and a predetermined strength (TS of 590 MPa or more) cannot be obtained.

[0028] That is, (C/Ti*) must satisfy the following expression (3) :

[0029] In the expressions (1) to (3), Ti, N, and C represent contents of corresponding elements (percent by mass).

2) Steel microstructure

[0030] The steel microstructure of the present invention will be described below.

[0031] In order to achieve a TS of 590 to 780 MPa, an El of 28% or more, and a λ of 100% or more, it is essential to form a steel microstructure mainly composed of a hard ferrite phase. By precipitating Ti carbides in a highly ductile ferrite phase during ferrite transformation, it is possible to obtain a steel sheet having high strength and high ductility. In order to suppress precipitation of cementite which adversely affects stretch flangeability, it is necessary to fix C contained as fine Ti carbides. Since cementite is very hard, it serves as an origin for generation of voids during blanking and during stretch flange forming. Generated voids grow and link together, which leads to fracture. However, in the steel sheet having a steel microstructure in which the area fraction of the ferrite phase is 95% or more, since the spacing between cementite grains is sufficiently large, development of linkage of voids can be slowed down even if cementite is contained, and stretch flangeability is satisfactory compared with the case where the area fraction of ferrite is less than 95%. Furthermore, when the area fraction of the ferrite phase is 95% or more, it is possible to achieve an El of 28% or more.

[0032] As long as the area fraction of the ferrite phase is 95% or more, even if at least one of a martensite phase, a bainite phase, and a pearlite phase is contained as a secondary phase, the advantages of the present invention is not impaired.

[0033] In order to obtain a steel sheet having high strength and uniformity of material, in addition to satisfying the condition that the area fraction of the ferrite phase is 95% or more, it is necessary to set the ferrite grain size and the size of Ti carbides to be fine and uniform. Furthermore, it is necessary to obtain as many Ti carbides as possible. Specifically, as long as the average ferrite grain size is 10 µm or less, the average grain size of Ti carbides is 10 nm or less, and Ti in the amount of 80% or more of Ti* (the amount of Ti that can form Ti carbides) is precipitated as Ti carbides, it is possible to achieve a TS of 590 MPa or more and a ΔTS of 15 MPa or less.

3) Manufacturing conditions

[0034] The manufacturing conditions of the present invention will be described.
Slab heating temperature: 1,200°C to 1,300°C

[0035] In order to precipitate fine Ti carbides in the ferrite phase after hot rolling, it is necessary, before hot rolling, to dissolve coarse Ti carbides precipitated in the slab. For that purpose, the slab needs to be heated at 1,200°C or higher. On the other hand, heating at higher than 1,300°C increases the amount of scales formed, resulting in a decrease in yield. Therefore, the slab heating temperature is set at 1,200°C to 1,300°C.
Hot rolling finishing temperature: 900°C or higher

[0036] Since the content of Mn, which is an austenite former, is low, the Ar₃ point is relatively high. Specifically, a finishing temperature of lower than 900°C causes coarsening of ferrite grains and an abnormal microstructure, resulting in a decrease in strength and uniformity of material. Therefore, the finishing temperature is set at 900°C or higher.
Cooling start time after hot rolling: within 2 seconds
Average cooling rate during primary cooling after hot rolling: 30°C/s or more

[0037] When the time until the start of primary cooling after hot rolling exceeds 2 seconds, coarse ferrite grains and coarse Ti carbides are formed, resulting in a decrease in strength and uniformity of material. Therefore, the cooling start time after hot rolling is set to be within 2 seconds. For the same reason, the average cooling rate during primary cooling after hot rolling is set at 30°C/s or more.
Primary cooling stop temperature: 650°C to 750°C

[0038] By stopping primary cooling in a temperature range of 650°C to 750°C, it is necessary to promote ferrite transformation and formation of fine Ti carbides during subsequent natural cooling (air cooling). When the cooling stop temperature is lower than 650°C, ferrite is not formed sufficiently, an area fraction of 95% or more cannot be secured, and it is not possible to precipitate Ti in the amount of 80% or more of Ti* as Ti carbides. On the other hand, when the cooling stop temperature exceeds 750°C, ferrite grains and Ti carbides are coarsened, and it is difficult to achieve a ferrite grain size of 10 µm or less and an average grain size of Ti carbides of 10 nm or less. Therefore, the primary cooling stop temperature is set at 650°C to 750°C.
Air cooling time after primary cooling: 5 to 20 seconds

[0039] When the air cooling time is less than 5 seconds, the ferrite phase is not formed sufficiently, and it is difficult to achieve an area fraction of the ferrite phase of 95% or more and to precipitate Ti in the amount of 80% or more of Ti* as Ti carbides. When the air cooling time exceeds 20 seconds, ferrite grains and Ti carbides are coarsened, and it is difficult to achieve a ferrite grain size of 10 µm or less and an average grain size of Ti carbides of 10 nm or less. Therefore, the air cooling time after primary cooling is set at 5 to 20 seconds.
Secondary cooling condition: average cooling rate 30°C/s or more

[0040] In order to maintain a ferrite grain size of 10 µm or less and an average grain size of Ti carbides of 10 nm or less obtained by combination of primary cooling after hot rolling and the air cooling step, it is necessary to perform secondary cooling at an average cooling rate of 30°C/s or more after the air cooling until coiling.
Coiling temperature: 560°C or lower

[0041] In the manufacturing method of the present invention, the microstructure of the steel sheet and the state of Ti carbides are determined before coiling, and then a coiling process is performed. However, when the coiling temperature exceeds 560°C, Ti carbides are coarsened, and strength is decreased. Therefore, the coiling temperature is set at 560°C or lower. From the viewpoint of securing good steel sheet shape, the coiling temperature is preferably set at 350°C or higher.

[0042] Regarding other manufacturing conditions, usual conditions may be used. For example, steel having a desired chemical composition is produced by refining in a converter, electric furnace, or the like, and then secondary refining in a vacuum degassing furnace. Subsequent casting is desirably performed by a continuous casting process from the viewpoint of productivity and quality. After casting, hot rolling is performed in accordance with the method of the present invention. After hot rolling, the properties of the steel sheet are not impaired even in the state in which scales are attached to the surface or in the state in which scales are removed by pickling. Furthermore, after hot rolling, it is also possible to perform temper rolling, hot dip zinc-based plating, electrogalvanizing, or chemical conversion treatment. The term "zinc-based plating" refers to plating using zinc or zinc as a main component (at a zinc content of 90% or more), for example, plating containing an alloying element, such as Al or Cr, in addition to zinc, or plating in which alloying treatment is performed after zinc-based plating is performed.

[EXAMPLES]

[0043] Steels A to H having the chemical compositions (compositions) shown in Table 1 were refined by a converter, and slabs were formed by a continuous casting process. The resulting steel slabs were heated at 1,250 °C, and coil-shaped, hot-rolled steel sheet Nos. 1 to 18 with a thickness 3.2 mm were produced under the hot rolling conditions shown in Table 2.

[0044] Note that, in Tables 1 and 2, underlines indicate that values are outside the ranges of the present invention.

[Table 1]

Steel Symbol	Chemical composition (mass%)										Remarks
	C	Si	Mn	P	S	Al	N	Ti	Ti*	C/Ti*
A	0.041	0.03	0.61	0.017	0.002	0.039	0.0012	0.120	0.116	0.354	Within the range of the invention
B	0.025	0.02	0.43	0.015	0.002	0.041	0.0012	0.086	0.082	0.305	Within the range of the invention
C	0.062	0.02	0.78	0.016	0.002	0.042	0.0009	0.169	0.166	0.374	Within the range of the invention
D	0.019	0.01	0.43	0.017	0.002	0.041	0.0025	0.110	0.102	0.187	Outside the range of the invention
E	0.077	0.02	0.64	0.015	0.002	0.040	0.0025	0.104	0.096	0.806	Outside the range of the invention
F	0.033	0.56	0.63	0.015	0.002	0.045	0.0035	0.108	0.096	0.343	Outside the range of the invention
G	0.038	0.02	1.25	0.018	0.002	0.045	0.0021	0.112	0.105	0.362	Outside the range of the invention
H	0.036	0.02	0.66	0.016	0.002	0.041	0.0045	0.075	0.060	0.603	Outside the range of the invention

[Table 2]

Hot-rolled steel sheet No.	Steel symbol	Finishing temperature	Primary cooling			Air cooling	Secondary cooling	Coiling temperature	Remarks
			Cooling start time after rolling	Average cooling rate	Cooling stop temperature	Time	Average cooling rate
		°C	s	°C/s	°C	s	°C/s	°C
1	A	920	1.5	110	700	10	50	500	Example of invention
2		910	1.5	110	650	15	60	500	Example of invention
3		910	1.5	110	750	7	60	400	Example of invention
4		910	3.0	110	700	10	55	500	Comparative example
5		920	1.5	20	700	10	50	500	Comparative example
6		920	1.5	110	600	20	50	500	Comparative example
7		920	1.5	110	800	10	55	450	Comparative example
8		910	1.5	110	700	25	60	550	Comparative example
9		910	1.5	110	700	10	20	500	Comparative example
10		920	1.5	110	700	10	55	600	Comparative example
11	B	910	1.5	110	700	10	50	500	Example of invention
12		880	1.5	110	700	10	50	500	Comparative example
13	C	920	1.5	110	700	10	50	500	Example of invention
14	D	910	1.5	110	700	10	50	500	Comparative example
15	E	920	1.5	110	700	10	55	500	Comparative example
16	F	920	1.5	110	700	10	60	500	Comparative example
17	G	910	1.5	110	700	10	50	500	Comparative example
18	H	920	1.5	110	700	10	55	500	Comparative example

[0045] In each of the coils, which had been pickled, after trimming innermost and outermost turns and both ends in the coil width direction by 10 mm, the coil was divided into 20 equal portions in the longitudinal direction of the coil and into 8 equal portions in the width direction. JIS No. 5 tensile test specimens were taken, in a direction parallel to the rolling direction, from 189 positions including trimmed coil ends. A tensile test was carried out in accordance with JIS Z 2241, at a cross head speed of 10 mm/min. The average tensile strength (TS) and total elongation (El), and, as a measure of uniformity of material, the variation in TS in the trimmed coil, i.e., the standard deviation of TS (ΔTS) were obtained.

[0046] Furthermore, hole expanding test specimens were taken from 189 positions, and a hole expanding test was carried out in accordance with The Japan Iron and Steel Federation standard JFST1001. Thus, the average hole expanding ratio λ was obtained. Regarding the area fractions of the ferrite phase and the secondary phase in the entire microstructure, test specimens for a scanning electron microscope (SEM) were taken from 189 positions. A cross section in the thickness direction parallel to the rolling direction of each test specimen was polished and then etched with nital. SEM photographs were taken at a magnification of 1,000 times for 10 viewing fields in the vicinity of the central part in the thickness direction. The ferrite phase and phases other than the ferrite phase, such as the martensite phase, were identified by image processing. The areas of the ferrite phase and phases other than the ferrite phase, such as the martensite phase, were measured by image analysis, and the proportion (percentage) in the area of the viewing field was obtained. The area fraction of the ferrite phase was defined by the lowest value in 189 points.

[0047] The average ferrite grain size was determined by the intercept method from the 10 viewing fields of the SEM photographs. That is, three vertical lines and three horizontal lines were drawn in each SEM photograph, and the ferrite grain intercept length was obtained. The value obtained by multiplying the resulting grain intercept length by 1.13 (corresponding to the nominal grain size according to ASTM) was defined as the ferrite grain size, and the average ferrite grain size was obtained by averaging the grain sizes in the 10 viewing fields.

[0048] The maximum value of the average ferrite grain sizes obtained in the 189 positions is shown in Table 3 below. Regarding the average grain size of Ti carbides, thin films were taken by the twin jet method from 21 positions, i.e., 20 equal portions divided in the longitudinal direction of the coil including coil ends, in the central part in the coil width direction and in the central part in the thickness direction. Observation was performed using a transmission electron microscope (TEM). The grain sizes of 3,000 or more Ti carbide grains were measured by image analysis, and the average value was obtained. Regarding the amount of Ti carbides precipitated, for the 21 positions from which specimens for TEM observation were taken, about 0.2 g was subjected to constant-current electrolysis in a 10% AA-based electrolyte solution (10vol% acetylacetone-1mass% tetramethylammonium chloride-methanol) at a current density of 20 mA/cm², to extract Ti carbides. By analyzing the extracted amount, the amount of Ti carbides precipitated was determined.

[0049] The results are shown in Table 3. Underlines in the table indicate that values are outside the ranges of the present invention.

[0050] In Table 3, Steel sheet Nos. 1 to 3, 11, and 13 are examples of the invention, and Steel sheet Nos. 4 to 10, 12, and 14 to 18 are comparative examples.

[0051] The ferrite area fraction is shown in Table 3. Note that the phase other than ferrite was a pearlite or bainite phase.

[Table 3]

Hot-rolled steel sheet No.	Mechanical properties				Microstructure				Remarks
	TS	ΔTS	El	λ	Ferrite area fraction	Ferrite grain size	Average grain size of Ti carbides	Ratio of amount of Ti carbides precipitated to amount of Ti*
	MPa	MPa	%	%	%	µm	nm	%
1	698	7	30	112	97	7	6	86	Example of invention
2	677	12	29	107	98	7	5	81	Example of invention
3	613	11	31	116	98	10	9	96	Example of invention
4	586	28	29	109	96	11	8	97	Comparative example
5	565	31	31	111	98	12	9	94	Comparative example
6	621	35	26	78	76	6	6	76	Comparative example
7	532	47	27	64	61	9	12	64	Comparative example
8	578	21	29	104	98	9	11	93	Comparative example
9	574	27	29	107	96	13	7	94	Comparative example
10	564	22	31	108	97	11	11	98	Comparative example
11	602	8	31	118	96	8	7	89	Example of invention
12	578	19	31	105	97	11	7	79	Comparative example
13	773	14	28	103	97	7	6	87	Example of invention
14	549	11	28	121	99	8	4	59	Comparative example
15	688	10	29	67	82	7	7	88	Comparative example
16	595	25	32	101	98	11	7	77	Comparative example
17	702	18	26	86	76	6	6	71	Comparative example
18	574	13	30	113	83	7	6	94	Comparative example

[0052] In each of Nos. 1 to 3, 11, and 13, which are examples of the invention, TS is 590 to 780 MPa, El is 28% or more, λ is 100% or more, thus showing high strength and excellent ductility and stretch flangeability, and the variation in TS (ΔTS) is 15 MPa or less, showing a small variation in strength in the coil and excellent uniformity of material.

[0053] On the other hand, in No. 4, which is a comparative example, although the steel type is A and the composition is within the range of the present invention, the primary cooling start time after rolling is 3.0 seconds, which exceeds 2 seconds, and thus the manufacturing condition is outside the range of the present invention. For this reason, the ferrite grain size is 11 µm, showing coarsening, TS is 586 MPa, showing low strength, and ΔTS is 28 MPa, showing poor uniformity of material.

[0054] In No. 5, which is a comparative example, although the steel type is A and the composition is within the range of the present invention, the average cooling rate during primary cooling after rolling is 20°C/s, which is less than 30°C/s, and thus the manufacturing condition is outside the range of the present invention. For this reason, as in No. 4, the ferrite grain size is 12 µm, showing coarsening, TS is 565 MPa, showing low strength, and ΔTS is 31 MPa, showing poor uniformity of material.

[0055] In No. 6, which is a comparative example, although the steel type is A and the composition is within the range of the present invention, the cooling stop temperature in primary cooling after rolling is 600°C, which is lower than 650°C, and thus the manufacturing condition is outside the range of the present invention. For this reason, the ferrite phase is not sufficiently formed, the ferrite area fraction is low at 76%, the amount of Ti carbides precipitated is 76% of Ti*, which is short of 80%, El is slightly low at 26%, λ is slightly low at 78%, and in particular, ΔTS is 35 MPa, showing poor uniformity of material.

[0056] Furthermore, in No. 7, which is a comparative example, although the steel type is A and the composition is within the range of the present invention, the cooling stop temperature in primary cooling after rolling is 800°C, which is higher than 750°C, and thus the manufacturing condition is outside the range of the present invention. For this reason, the average grain size of Ti carbides is 12 nm, which exceeds 10 nm, and the amount of Ti precipitated is 64% of Ti*, which is less than 80%. Furthermore, the ferrite area fraction is 61%, which is less than 85%. Consequently, TS is low at 532 MPa, and ΔTS reaches 47 MPa, thus showing low strength and poor uniformity of material. Furthermore, El is 27% and λ is 64%, thus showing poor ductility and stretch flangeability.

[0057] In No. 8, which is a comparative example, although the steel type is A and the composition is within the range of the present invention, the air cooling time after primary cooling is 25 seconds, which exceeds 20 seconds, and thus the manufacturing condition is outside the range of the present invention. For this reason, the average grain size of Ti carbides is 11 nm, showing coarsening. Consequently, TS is 578 MPa, and ΔTS is 21 MPa, showing slightly poor strength and uniformity of material.

[0058] In No. 9, which is a comparative example, although the steel type is A, which is within the range of the present invention, the average cooling rate in secondary cooling is 20°C/s, which is lower than 25°C/s, deviating from the manufacturing condition of the present invention. For this reason, the ferrite grain size is 13 µm, showing coarsening. Consequently, TS is 574 MPa, and ΔTS is 27 MPa, showing slightly poor strength and uniformity of material.

[0059] In No. 10, which is a comparative example, although the steel type is A, which is within the range of the present invention, the coiling temperature is 600°C, which is higher than 560°C, deviating from the manufacturing condition of the present invention. The average grain size of Ti carbides and the ferrite grain size exceed 10 nm and 10 µm, respectively, showing coarsening. Consequently, TS is 564 MPa, and ΔTS is 22 MPa, showing slightly poor strength and uniformity of material.

[0060] In each of No. 11, which is an example of the invention, and No. 12, which is a comparative example, the steel type is B, and the composition is within the range of the present invention. In No. 11, which is an example of the invention, the hot rolling finishing temperature is 910°C, satisfying the manufacturing condition of the present invention. In contrast, in No. 12, which is a comparative example, the hot rolling finishing temperature is 880°C, deviating from the manufacturing condition of the present invention. For this reason, in comparative example 12, the ferrite grain size is 11 µm, showing coarsening, resulting in poor strength and uniformity of material.

[0061] In No. 14, which is a comparative example, the steel type is D, in which the C content is 0.019% and the (C/Ti*) value is 0.187, and the composition deviates from the conditions of the present invention. For this reason, TS is 549 MPa, showing low strength.

[0062] In No. 15, which is a comparative example, the steel type is E, in which the C content is 0.077% and the (C/Ti*) value is 0.806, and the composition deviates from the conditions of the present invention. For this reason, λ is 67%, showing poor formability.

[0063] In No. 16, which is a comparative example, the steel type is F, in which the Si content is 0.56%, and the composition deviates from the condition of the present invention (0.1% or less). For this reason, the ferrite grain size is 11 µm, exceeding 10 µm, and ΔTS is 25 MPa, showing poor uniformity of material.

[0064] In No. 17, which is a comparative example, the steel type is G, in which the Mn content is 1.25%, and the composition deviates from the condition of the present invention (less than 0.80%). Furthermore, the ratio of the amount of Ti carbides precipitated to the amount of Ti* is low at 0.71, falling below the condition of the present invention. For this reason, the ferrite area fraction is low, ΔTS is 18 MPa, showing poor uniformity of material, El is 26%, and λ is 86%, showing poor ductility and stretch flangeability.

[0065] In No. 18, which is a comparative example, the steel type is H, in which the Ti content is 0.075%, and the composition deviates from the condition of the present invention (0.08% to 0.16%). Furthermore, Ti* is 0.060, which is less than 0.08, and (C/Ti*) is 0.603, which is more than 0.375, both of which deviate from the conditions of the present invention. For this reason, TS is 574 MPa, showing poor strength.

[0066] As described above, in the present invention, it is possible to obtain a hot-rolled steel sheet having a TS of 590 to 780 MPa, an El of 28% or more, a λ of 100% or more, and a ΔTS of 15 MPa or less, thus having excellent ductility (elongation property) and stretch flangeability and excellent uniformity of material.

Claims

1. A high strength hot-rolled steel sheet having excellent ductility, stretch flangeability, and uniformity of material, characterized in that the steel sheet has a steel composition comprising:

in percent by mass,

0.020% to 0.065% of C;

0.1% or less of Si;

0.40% to less than 0.80% of Mn;

0.030% or less of P;

0.005% or less of S;

0.08% to 0.20% of Ti;

0.005% to 0.1% of Al;

0.005% or less of N; and

the balance being Fe and incidental impurities, in which Ti* specified by the expression (1) below satisfies the expressions (2) and (3) below,

and the steel sheet has a steel microstructure including, in terms of area fraction, 95% or more of a ferrite phase and the balance being at least one of a pearlite phase, a bainite phase, and a martensite phase; the average ferrite grain size is 10 µm or less; the average grain size of Ti carbides precipitated in steel is 10 nm or less; and Ti in the amount of 80% or more of Ti* is precipitated as Ti carbides:






where Ti, N, and C represent contents of corresponding elements (percent by mass).

2. A method for manufacturing a high strength hot-rolled steel sheet characterized by comprising: heating a steel slab having the steel composition according to Claim 1 at a temperature in a range of 1,200°C to 1,300°C; then performing hot rolling at a finishing temperature of 900°C or higher; starting cooling within 2 seconds after the hot rolling at a cooling rate of 30 °C/s or more; stopping the cooling at a temperature of 650°C to 750°C; subsequently, after undergoing a natural cooling step for 5 to 20 seconds, performing cooling at a cooling rate of 30°C/s or more; and performing coiling in a coil shape at 560°C or lower.